Door, hood, and trunk deck lids of vehicles have been formed of one unitary outer skin of sheet metal joined around its periphery to a second inner reinforcing panel of sheet metal by hemming a generally upturned flange along each edge of the outer sheet over an adjacent edge of the inner panel.
This hemming has been accomplished in two separate stages. Prior to performing the first stage, the reinforcing panel is nested within the outer panel fixtured on an anvil die on a base of a prehemming machine. Upon fixturing the assembly, a tool of the machine, commonly referred to as a hemming steel, engages and bends an edge of the outer panel to an acute included angle with respect to the outer panel. After the prehemming of all edges to be joined, both panels are released, transferred to and fixtured in a second hemming machine where a second tool completely bends the prehemmed edge of the outer panel over the peripheral edge of the reinforcing panel to secure and attach the panels together as a unitary structural member for assembly on a vehicle.
Typically, a plurality of both prehemming and final hemming machines are respectively grouped around the periphery of a panel to perform all prehemming and hemming operations for one assembly either sequentially or substantially simultaneously. This type of hemming process and equipment has proven to be commercially successful and is still in widespread use.
However, this hemming process has disadvantages. Such a two stage hemming process is costly and inefficient by requiring multiple components, namely a prehemming machine, a transfer mechanism and a final hemming machine to perform the entire hemming assembly operation. Additionally, a considerable amount of tooling and transfer equipment is required for this type of process, it consumes a great deal of valuable manufacturing floor space and it increases the likelihood of equipment malfunction which can undesirably delay production. Furthermore, the process requires numerous steps to completely hem a single component. For example, the assembly must be fixtured, prehemmed, released, transferred, fixtured and final hemmed resulting in a low finished part production rate. Finally, this two stage process requires a relatively larger sheet flange depth which increases component weight and cost.
This two stage process is also susceptible to quality control problems. During transfer to the final hemming station, the panels may loosen from each other, become skewed with respect to each other, or not be properly located with respect to the final hemming station resulting in a finished hemmed assembly of lesser quality and poor structural integrity. An assembly with these characteristics may have to be repaired or scrapped, thereby increasing production costs and lowering profits. Even worse, an ill-assembled structural member with these flaws, if incorporated into an assembled vehicle may fit poorly and affect perceived quality by prospective purchasers, thereby reducing vehicle sales and profits. An assembled defective structural member may further lose integrity as the vehicle is subjected to road vibration during use and possibly require replacement and negatively impact an owners' future vehicle purchasing decision.
More recently, hemming machines have been designed which perform both the prehem and final hem operation in a single machine tool station which eliminates the need for a complex transfer mechanism. Hemming machines of this type vary in the kind of mechanism used and the manner of carrying out the hemming operations. Representative of these hemming machines are U.S. Pat. Nos.: Kollar et. al. 3,191,414; E. R. St. Denis 3,276,409; Dacey Jr. 4,706,489; and Dacey Jr. 5,083,355.
The hemming machines embodied in the Kollar '414 and E. R. St. Denis '409 patents are of similar construction and operation. Both patents disclose a pair of fluid powered drives carried by a frame of the machine for driving a single hemming steel through both the prehem and final hemming stages. Each machine utilizes one drive to control the sideward motion of the hemming tool toward the anvil and sheet during the prehem operation and a second drive for downwardly moving the tool to clinch the flange in a hem overlapping the structural reinforcing panel.
A disadvantage of the these single station prior art machines is that the hemming tool or steel continuously contacts the sheet edge during both stages of bending the flange which may produce undesirable distortion and highlighting in the sheet. A further drawback is that failure to maintain precise actuation sequencing of the first and second drives during hemming may result in the outer panel being defectively hemmed to the reinforcing panel causing the costly scrapping of the assembly. Furthermore, they have an abrupt motion of the hemming steel due to cam drives and high actuation forces. Also, the equipment to accurately sequence the actuation of each drive adds to the complexity of the machine, requiring additional costly maintenance while reducing reliability. Finally, the sequencing complexity of this type of hemmer limits the number of assemblies which may be produced during a given period of time.
Dacey Jr. '489 discloses a hemming machine utilizing a single drive and hemming steel connected by a complicated system of linkages and a cam and follower arrangement to perform both the prehem and final hem operations. Dacey Jr. '355 discloses a hemming machine having dual drives and a single hemming steel connected by a linkage and eccentric shaft arrangement to perform both the prehem and final hem operations.
A shortcoming of these prior art machines is that the hemming tool follows an arcuate sideways path, literally "wiping" the flange while prehemming the sheet edge which can introduce unwanted distortion or highlights in the outer panel adjacent the hem which are visual even after finishing and painting it. Moreover, the outer panel bends immediately adjacent the edge of the inner panel rather than at a predetermined desired break point which results in undesirable variations and inconsistencies from one panel assembly to another. The drive and sequencing mechanism is also complicated and requires frequent and costly production-delaying adjustments and is prone to unacceptable wear limiting the machines commercial usefulness.